The invention concerns a magnetic resonance probe head, comprising                a vacuum container in which several RF resonator coils are disposed, which can be cryogenically cooled and which are each designed as planar coils disposed parallel to a z direction, wherein the overall RF resonator coils have a longer extension in the x direction than in the y direction, the x, y, z directions forming a rectangular coordinate system,        a central tube block disposed between the RF resonator coils and having a recess for a test sample, which is elongated in the z direction, wherein the central tube block partially delimits the vacuum container, and the recess is located outside of the vacuum container.        
A magnetic resonance probe head of this type is disclosed e.g. in ref. [5].
Nuclear magnetic resonance (NMR) methods are used to analyze sample compositions or determine the structure of substances in samples. In these NMR methods, the sample is exposed to a strong static magnetic field B0 in a z direction, and high-frequency electromagnetic pulses, which are orthogonal thereto, are irradiated into the sample in the x or y direction, causing an interaction with the nuclear spins of the sample material. The temporal development of these nuclear spins of the sample generates, in turn, high-frequency electromagnetic fields which are detected in the NMR apparatus. The detected radio frequency (RF) fields provide information about the properties of the sample.
The sensitivity of high-resolution NMR spectroscopy has been considerably increased in recent years by using cooled magnetic resonance probe heads. This is true, in particular, for high field NMR, i.e. NMR with static magnetic fields of more than 7 T, in particular, more than 11 T. The receiver coils and the receiver electronics of these probe heads are thereby cooled down to cryogenic temperatures (below 100 K). This reduces the thermal noise of resistive elements. Cooling also reduces the RF (radio frequency) resistance of metals leading to an increase of the Q value of RF resonator coils in the NMR probe head.
One constructive problem of cooled probe heads is the temperature control of the test sample which should generally be kept close to room temperature (−40 to 200° C., typically around 20° C.). Ref. [3] discloses a probe head, wherein a substantially circular cylindrical jacket-shaped RF resonator coil, which is to be cooled, is mounted to a cooled platform and is disposed in a vacuum container (also called a vacuum dewar). The vacuum dewar has a circular cylindrical recess which penetrates through the inside of the RF resonator coil and into which a test sample, e.g. a round test tube, which is filled with a sample substance to be investigated, is disposed. A ventilation gap (temperature control gap) remains between the wall of the test tube and the dewar wall of the recess. The ventilation gap adjusts a temperature control gas flow (e.g. air or nitrogen) that brings the test sample to a desired temperature. The test sample must thereby be carefully centered in order to prevent uneven heat input or irregular heat discharge. An insulation gap which prevents formation of a heat bridge remains between the dewar wall of the recess and the RF resonator coil. The dewar wall of the recess which surrounds the test sample is also called the central tube.
In addition to the Q value (i.e. the electric resistance) and the temperature of the receiver coil, the sensitivity of an NMR probe head also depends on the efficiency (field per unit current in the measuring volume) or the filling factor (useful energy in the measuring volume divided by the total energy). The larger the fraction of the measuring volume relative to the coil volume, the better is the efficiency/filling factor. Although the filling factor is a common value in literature, its quantitative use is problematic, since the behavior during scaling of the test sample is different from that during scaling of the coil. Since this value is very transparent, it is also used herein.
In particular, in the probe head of Ref. [3], the ventilation gap and the dewar wall thickness limit the achievable efficiency or filling factor. The dewar wall has a minimum thickness due to manufacturing and mechanical reasons, irrespective of the size of the recess or the size of the test sample. The temperature control gap must also have a minimum width in order to ensure sufficient temperature control gas flow, which is also substantially independent of the size of the test sample. In particular, for test samples having a small diameter (smaller than 5 mm), the ventilation gap and dewar wall occupy a considerable part of the coil volume, such that test samples having a small diameter only achieve a small efficiency/filling factor.
In order to compensate for the small efficiency/filling factor of small test samples, superconducting receiver coils of high-temperature superconducting material (HTS) are conventionally used. HTS receiver coils have a considerably higher Q value than comparable metal coils. However, HTS receiver coils for magnetic resonance probe heads can currently only be produced on planar substrates. These planar receiver coils project past the central tube transverse to the extension thereof, and therefore have a worse efficiency/filling factor than circular cylindrical jacket shaped coils. The poor efficiency/fill factor is, however, overcompensated for by the higher Q value of the planar HTS resonator coils.
Ref. [5] discloses a configuration with two parallel, opposite HTS RF resonator coils, between which a round central tube for a test sample with round test tube is disposed. The RF resonator coils are approximately 2.5 times wider than the central tube. The wall thickness of the dewar wall and the width of the air gap between the test sample and the dewar wall of this configuration also limit the separation between the two planar RF resonator coils and thereby the efficiency/filling factor.
In contrast thereto, it is the underlying purpose of the present invention to present a magnetic resonance probe head, which improves the sensitivity, in particular, for small and round test samples. This is important, in particular, for high-resolution and high-field NMR.